1. Field of the Invention
This invention relates to a scale for use for measurement of the displacement of an object to be examined and to a displacement measuring apparatus. More particularly, the invention relates to a scale and a measuring apparatus which display their effects when it is desired to highly accurately measure the displacement of an object to be examined to obtain, for example, the amount of movement and the speed of movement of such object.
2. Related Background Art
Heretofore, a linear encoder has often been used for the detection of the position and amount of movement of an object to be measured such as an X-Y stage. Particularly, optical type linear encoders have been used in various fields because of their ability to accomplish highly accurate measurement of displacement.
FIG. 1 of the accompanying drawings schematically shows the construction of an optical type linear encoder according to the prior art. In FIG. 1, the reference numeral 61 designates a scale provided with a division 61a. The reference numeral 68 denotes detecting means provided therein with a light-emitting element and a light receiving sensor for receiving the light from the division 61a and reading the division 61a. The reference numeral 69 designates an object which is a movable stage movable in a predetermined direction as indicated by arrow A. The scale 61 is attached to the movable stage 69, and is moved with the movement of the movable stage 69 relative to the detecting means 68 fixed to a base plate, not shown.
The linear encoder shown in FIG. 1 detects the displacement of the division 61a on the scale 61 resulting from the movement of the movable stage 69, by the detecting means 68, thereby detecting the amount of displacement of the movable stage 69.
The division 61a comprises light-transmitting portions and light-intercepting portions both having a slit-like shape, and these portions are alternately arranged at a predetermined pitch in a direction orthogonal to the lengthwise direction of the slit (the widthwise direction). The light from the aforementioned light-emitting element irradiates an area including several light-transmitting portions and light-intercepting portions, and the aforementioned light receiving sensor photoelectrically converts the light passed through the light-transmitting portions.
FIG. 2 of the accompanying drawings is an illustration showing a state in which the direction of movement A of the movable stage 69 and the direction of arrangement of the portions of the division 61a of the scale 61 are not coincident with each other but are inclined, and FIG. 3 of the accompanying drawings is an enlarged view showing the state of the division 61a of the scale 61 in the state shown in FIG. 2.
If as shown in FIGS. 2 and 3, the direction of movement A of the movable stage 69 and the direction of arrangement B of the portions of the division 61a of the scale 61 are not coincident with each other, the output signal from the detecting means 18 will be as follows: EQU .epsilon.=L2-L1 EQU L1=L2 cos .theta.
.epsilon.=L2(1-cos .theta.)
where .eta. is the angle formed between the direction A and the direction B, L1 is the read value of the division 61a of the scale 61 when the movable stage 69 is displaced, L2 is the actual amount of movement of the movable stage 69, and .epsilon. is the difference between L1 and L2.
Accordingly, assuming that L2=100 mm and .theta.=0.08.degree., there occurs an error of .epsilon.=0.1 .mu.m, and simply because the division is inclined by only 0.08.degree. with respect to the direction of movement of the object to be examined, accurate measurement in the unit of submicron becomes impossible.